532 IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-21, NO. 2, MARCH 1985

High-T, S u p e r c o n d u c t i n g I n t e g r a t e d C i r c u i t : A dc SQUID w i t h I n p u t C o i l

M. S. D i I o r i o and M.R. Beasley Depar tment o f App l ied Phys ics

S t a n f o r d U n i v e r s i t y S tan ford , CA 94305

ABSTRACT

We have f a b r i c a t e d a h i g h t r a n s i t i o n t e m p e r a t u r e s u p e r c o n d u c t i n g i n t e g r a t e d c i r c u i t c o n s i s t i n g o f a dc SQUID and an i n p u t c o u p l i n g c o i l . The purpose i s t o ascer ta in the gener ic p rob lems assoc ia ted w i th con- s t r u c t i n g a h i g h - T c i r c u i t as w e l l as t o f a b r i c a t e a high performance 8c SQUID. The superconductor used f o r b o t h t h e SQUID and t h e i n p u t c o i l i s Nb$n which must be d e p o s i t e d a t 800°C. I m p o r t a n t l y , t h e i n s u l a - t o r s e p a r a t i n g SQUID and i n p u t c o i l m a i n t a i n s i t s i n t e g r i t y a t t h i s e l e v a t e d t e m p e r a t u r e . A h o l e i n t h e i n s u l a t o r p e r m i t s c o n t a c t t o t h e i n n e r m o s t w i n d i n g o f t h e c o i 1. Th is con tac t has been ach ieved w i thou t s i g n i f i c a n t d e g r a d a t i o n o f t h e s u p e r c o n d u c t i v i t y . Consequent ly , the dev ice operates over a wide temperature range, from below 4.2 K t o near Tc.

I. INTRODUCTION

H igh t rans i t i on t empera tu re (h igh -T ) super- c o n d u c t i n g c i r c u i t s h o l d g r e a t t e c h n o l o g i c a f i n t e r e s t b e c a u s e o f t h e i r h i g h e r o p e r a t i n g t e m p e r a t u r e and t h e i r p o t e n t i a l l y s u p e r i o r p e r f o r m a n c e a t l o w tempera tu re . Opera t i on a t h ighe r t empera tu res pe rm i t s t h e u s e o f r e l a t i v e l y i n e x p e n s i v e c l o s e d - c y c l e r e f r i g e r a t i o n . A t l ower t empera tu res t he p r imary b e n e f i t s tems f rom the la rge I c R p r o d u c t ( c r i t i c a l c u r r e n t t i m e s d e v i c e r e s i s t a n c e ) w h i c h i s an o v e r a l l f i g u r e o f m e r i t f o r any superconduct ing dev ice and w h i c h s p e c i f i c a l l y d e t e r m i n e s t h e i n t r i n s i c e n e r g y s e n s i t i v i t y . M a t e r i a l p r o b l e m s have h i s t o r i c a l l y p revented the deve loym5nt o f h igh-Tc dev ices . Us ing a step-edge technique, 9 however, it has been p o s s i b l e t o i n c o r p o r a t e t h e A-15 type superconductors i n t h i n film high-Tc superconductor-normal metal-supercon- d u c t o r (SNS) dc SQUIDS which operate over a wide temperature range. To ' incorporate such SQUIDs i n t o p r a c t i c a l s u p e r c o n d u c t i n g c i r c u i t s , an i n p u t c o i l i s r e q u i r e d f o r e f f e c t i v e c o u p l i n g .

I n t h i s paper we desc r ibe t he des ign and f a b r i c a t i o n of a complete high-Tc+ dc SQUID c i r c u i t wh ich incorpora tes an i n t e g r a t e d h i g h - T p l a n a r i n p u t c o i l . A t t h e same t i m e t h i s work a l s o i e m o n s t r a t e s a general approach t o t h e f a b r i c a t i o n o f h i g h - T c i n t e g r a t e d c i r c u i t s .

11. DESIGN

F o r t h e d e s i g n o f t h e c i r c u i t we u t i l i z e a p l a n a r c o u p l i n g scheme t t has been adapted from t h a t o f Jaycox and K e t c h e p t o meet t h e s p e c i f i c c o n s t r a i n t s

a simplified schematic of our approach. Note t h a t t h e o f high-T, superconduct ing mater ia ls . F i g u r e 1 shows

c i r c u i t i s p h y s i c a l l y i n v e r t e d ( i . e . i n p u t c o i l on bottom, ground plane on t o p ) f r o m t h e c o n v e n t i o n a l c o n f i g u r a t i o n . The reason f o r t h i s will become c l e a r below, but it e s s e n t i a l l y stems f rom the need t o

temperatures. The i n p u t c o i l f o r m s t h e b o t t o m l e v e l depos i t h igh -Tc superconductors a t e l e v a t e d

and i s covered by an i n s u l a t i n g l a y e r w h i c h s e r v e s a dua l purpose. F i rs t , i t i s o l a t e s t h e c o i l f r o m t h e t o p l e v e l , w h i c h c o n t a i n s t h e SQUID. Second, t h i s i n s u l a t i n g l a y e r a l s o c o n t a i n s a step-edge necessary f o r t h e f a b r i c a t i o n o f t h e SNS mic robr idges wh ich

Manuscr ipt received September 10, 1984.

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F i g . 1. F a b r i c a t i o n scheme. (a ) Exp loded v iew w i th i npu t co i l on t h e b o t t o m , i n s u l a t o r i n t h e m i d d l e , and Cu/Nb3Sn b i l aye r con ta in ing g round p lane , coup l i ng hole, and dc SQUID, on the t op . (b ) Top v i e w o f c o m p l e t e d c i r c u i t .

0018-9464/85/0300-0532%01.00@1985 IEEE

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compr ise the dc SQUID. The t o p l e v e l i s a super- conductor -normal meta l b i layer f rom wh ich i s formed b o t h t h e dc SQUID and t h e g r o u n d p l a n e t o t h e c o i l . As i n t h e scheme o f Jaycox and Ketchen, the g round p l a n e a l s o p l a y s t h e r o l e o f t h e SQUID l o o p i t s e l f . The s i z e o f t h e h o l e i n t h e g r o u n d p l a n e t h e n de te rm ines t he se l f i nduc tance o f t he SQUID.

N o t e t h a t t h e i n s u l a t o r c o n t a i n s a c o n t a c t window wh ich pe rm i t s a s e c t i o n o f t h e t o p l e v e l t o be used as one o f t h e l e a d s t o t h e i n p u t c o i l . Hence i n a l l , o n l y t h r e e l a y e r s o f m e t a l l i z a t i o n and one l a y e r o f i n s u l a t i o n a r e needed. More impor tan t ly , on ly two layers o f superconductor , wh ich demand h i g h s u b s t r a t e temperatures, are requi red. The break i n t h e ground plane necessary t o f o r m t h e l e a d t o t h e i n p u t c o i 1 i s expected t9 have min imal impact on the coupl ing cons tan t , k , s i n c e t h e s c r e e n i n g c u r r e n t s f l o w n e a r t h e edges o f t h e s u p e r c o n d u c t o r . As will be descr ibed i n t h e n e x t s e c t i o n , t h e s t e p i n t h e i n s u l a t o r p r o - duces a very narrow break i n t h e s u p e r c o n d u c t i n g l a y e r ( a l o n g t h e step-ed,ge). This small break i n t h e ground p lane shou ld have m in ima l e f fec t on t he coup l i ng . I n a d d i t i o n , s i n c e a narrower s l i t r e s u l t s i n a lower i nduc tance , t he use o f t he s tep -edge a l l ows us t o l o c a t e t h e m i c r o b r i d g e s away f r o m t h e h i g h e r f i e l d r e g i o n n e a r t h e c o u p l i n g h o l e w i t h o u t i n t r o d u c i n g e x c e s s i v e p a r a s i t i c i n d u c t a n c e . N o t e t h a t t h e u s e o f a step-edge t o p roduce l ow i nduc tance s l i t s may be more g e n e r a l l y u s e f u l such as i n t h e f a b r i c a t i o n o f h i g h - T c t h i n film transformers .

S i n c e t h e n - t u r n i n p u t c o i l c o u p l e s t o t h e SQUID l oop i n t h e same f a s h i o n as a n : l t u r n t h i n film t r a n s f o mer, we can use the analysis of Jaycox and Ketchen' t o c a l c u l a t e t h e i m p o r t a n t c i r c u i t parameters. I n o r d e r t o m a x i m i z e t h e p e r f o r m a n c e o f t h e SQUID (i.e. t o m i n i m i z e t h e n o i s e ) , a l o w v a l u e o f t h e SQUID s e l f i n d u c t a n c e i s d e s i r e d . On t h e o t h e r hand, a h i g h e r v a l u e o f t h e s e l f i n d u c t a n c e i s d e s i r a b l e i n o r d e r t o i m p r o v e t h e c o u p l i n g t o t h e SQUID. I n o u r d e s i g n a compromise value o f a b o u t 100 pH i s chosen, which resuqts i n a c a l c u l a t e d c o u p l i n g h o l e d i a m e t e r o f 60 vm. The 1 i newid th o f t he 50 - t u r n i n p u t c o i l i s 5 p and t h e s p a c i n g i s a l s o 5 pm ( h e n c e t h e p i t c h i s 10 p). The ensu ing i npu t c o i l i n d u c t a n c e i s c a l c u l a t e d t o be 0.28 UH o f w h i c h 29 nH i s due t o t h e s t r i p l i n e i n d u c t a n c e o f t h e i n p u t c o i l w i t h r e s p e c t t o t h e SQUID loop. For purposes o f compar i son , 20 - tu rn co i l s were a l so f ab r i ca ted .

111. FABRICATION

The f a b r i c a t i o n p r o c e s s b e g i n s w i t h t h e e v a p o r a t i o n o f 3000 A o f Nb Sn on to a heated (800°C) sapph i re subs t ra te . The c o i ? i s t h e n f o r m e d by means o f c o n t a c t l i t h o g r a p h y and i o n beam e tch ing . Hence, we a r e l e f t w i t h t h e b o t t o m l e v e l i n F i g . l ( a ) . N e x t t h e i n s u l a t o r , S i 0 2 , i s s p u t t e r d e p o s i t e d on t o p o f t h e c o i l . A t o t a l o f 6000 A i s d e p o s i t e d i n t h r e e s e q u e n t i a l d e p o s i t i o n s o f 2000 A each. Between d e p o s i t i o n s t h e s a m p l e s a r e s u b j e c t e d t o u l t r a s o n i c a g i t a t i o n i n a c e t o n e i n o r d e r t o remove any p a r t i c l e s , a l o n g w i t h t h e i r o v e r l y i n g S i 0 2 . F o l l o w i n g t h i s , a s tep edge (1600 A i n h e i g h t ) i s i o n beam e t c h e d i n t o t h e Si02. I n a separate step, a s m a l l h o l e i s a l s o i o n beam e t c h e d i n t o t h e S i 0 d e f i n i n g a c o n t a c t window t o t h e i n n e r m o s t t u r n o i c ' t h e u n d e r l y i n g i n p u t c o i l ( s e e t h e m i d d l e l e v e l o f F i g . l ( a ) ) . The t o p l e v e l c o n s i s t s o f a Cu/Nb$n b i l a y e r and c o n t a i n s t h e two mic robr idges compr is ing the dc SQUID. T h i ? p rocess has been desc r ibed i n de ta i l e l sewhere . B r i e f l y , t h e s u p e r c o n d u c t o r , Nb3Sn (1000 A ) , i s e v a p o r a t e d a t an ang le so as t o produce a break i n t h e film a t t h e step-edge. T h i s e v a p o r a t i o n n u s t be c a r r i e d o u t a t 800°C i n o r d e r t o o b t a i n a high-Tc film. The sample i s t h e n r a p i d l y c o o l e d ( t o b e l o w

100°C) and the normal meta l , Cu (1000 A ) , i s evapora ted a t a complementary angle so as t o c o v e r t h e s t e p and j o i n t h e t w o s u p e r c o n d u c t i n g banks. The Cu must be d e p o s i t e d a t t h i s l o w e r t e m p e r a t u r e t o a v o i d agg lomera t i on and /o r i n te rd i f f us ion . Th i s requ i remen t t h a t t h e Cu n o t be exposed t o h i g h s u b s t r a t e t e m p e r a - t u r e s hence demands t h a t t h e Cu/Nb3Sn b i l a y e r be on t h e t o p a n d h e n c e t h e i n p u t c o i l be on the bottom. The t o p l e v e l , shown i n F i g . l ( a ) , i s c o m p l e t e d u s i n g s t a n d a r d p h o t o l i t h o g r a p h i c t e c h n i q u e s , a l o n g w i t h

F ig . 2 SEM m i c r o g r a p h o f i n n e r r e g i o n o f c i r c u i t (v iew i s reversed f rom F ig . 1 ) . Bo th s u p e r c o n d u c t i n g l a y e r s a r e Nb3Sn. A h o l e i n t h e i n s u l a t o r i n t h e b o t t o m l e f t p a r t o f p i c t u r e a l l o w s t h e t o p film superconductor t o make c o n t a c t t o u n d e r l y i n g 5 0 - t u r n i n p u t c o i 1. Mic robr idges a re about 4 ~ ~ 7 1 wide and l o c a t e d c l o s e r t h a n i s u s u a l t o t h e 60 um x 60 w c o u p l i n g h o l e . B o t h Cu and NbjSn f rom t h e b i l a y e r have been removed i n a swath cut a long t he s tep .

Fig. 3 SEM mic rograph o f i n n e r r e g i o n o f a d i f f e r e n t c i r c u i t . Here the top superconductor i s NbgGe w h i l e t h e 5 0 - t u r n i n p u t c o i l i s s t i l l Nb3Sn. The 2.5 um w ide m ic rob r idges a re now l o c a t e d f u r t h e r away f r o m t h e 5 m d i a m e t e r h o l e t o w h i c h t h e y a r e c o n n e c t e d v i a a l o w i n d u c t a n c e s l i t . O n l y t h e Cu has been removed f rom the swath cu t a long the s tep ( to n a r r o w t h e b r i d g e s ) .

534

s p u t t e r e t c h i n g o f t h e Cu and plasma e t c h i n g o f t h e Nb3Sn. The more i s o t r o p i c plasma e t c h i n g p r o c e s s i s p r e f e r r e d t o i o n beam e t c h i n g i n t h i s i n s t a n c e , because of problems due t o s h a d o w i n g t h a t r e s u l t f r o m t h e f a c t t h a t t h e u n d e r l y i n g s u r f a c e i s n o t p l a n a r . The w i d t h o f t h e m i c r o b r i d g e s i s d e f i n e d i n s u b s e q u e n t processing steps where 2-4 w w i d e b r i d g e s a r e f i r s t p a t t e r n e d u s i n g p r o j e c t i o n l i t h o g r a p h y and i o n beam e tch ing . These m ic rob r idges a re t hen na r rowed to t h e i r f i n a l w i d t h o f 0.2 m, by m a n s o f e l e c t r o n beam li hography fo l lowed by low angle ion beam etch ing. ' Here, the more d i rect ional ion beam e t c h i n g i s e s s e n t i a l due t o t h e s u b m i c r o n l i n e w i d t h s . The n o n - p l a n a r n a t u r e o f t h e u n d e r l y i n g s u r f a c e i s n o t a p rob lem here s ince the mic robr idges never need be l o c a t e d d i r e c t l y o v e r a c o i l w i n d i n g . A t o p v i e w o f t h e c o m p l e t e d c i r c u i t i s shown i n F i g . l ( b ) . SEM m i c r o g r a p h s o f t h e i n n e r r e g i o n o f t w o o f o u r c i r c u i t s a r e shown i n Fig. 2 and Fi.9. 3. (The p i c tu res a re reversed f rom F ig . 1.) N o t e t h a t t h e 2-4 pm wide br idges have not yet been narrowed t o t h e i r f i n a l width.

There are a number of general problems unique t o t h e f a b r i c a t i o n o f h i g h - T i n t e g r a t e d c i r c u i t s . The p r o c e s s j u s t o u t l i n e d d e a f s e f f e c t i v e l y w i t h many o f these, as w e l l as a d d r e s s i n g t h e s p e c i f i c p r o b l e m s o f o u r c i r c u i t .

One m a j o r d i f f i c u l t y l i e s i n m a i n t a i n i n g t h e i n t e g r i t y o f t h e i n s u l a t o r . M o s t h i g h - T c m a t e r i a l s r e q u i r e d e p o s i t i o n a t e l e v a t e d t e m p e r a t u r e s (e.9. 800°C f o r Nb3Sn and 900°C f o r Nb3Ge), w h i c h p u t s severe demands on t h e i n s u l a t i n g m a t e r i a l . I n a d d i - t i o n , t h e t o p o l o g y c r e a t e d by t h e u n d e r l y i n g i n p u t co i l r equ i res super io r s tep cove rage . Bo th t hese problems are so lved by t h e s p u t t e r d e p o s i t i o n o f Si02. The Si02. serves as an e x c e l l e n t i n s u l a t o r capab le o f su rv i v ing t o ve ry h igh t empera tu res and t he iso t rop ic depos i t ion p roduces the necessary coverage o f t h e c o i 1. The i n s u l a t o r must a l s o r e m a i n f r e e o f p i n h o l e s so as t o p r e v e n t s u p e r c o n d u c t i n g s h o r t s . The problem i s p a r t i c u l a r l y s e r i o u s i n o u r c a s e due t o t h e t e n d e n c y o f p a r t i c u l a t e m a t t e r b e n e a t h t h e d e p o s i t e d i n s u l a t i n g l a y e r t o come f r e e d u r i n g t h e subsequent h igh tempera ture fabr ica t ion s tep . Our s o l u t i o n l i e s w i t h t h e s e q u e n t i a l d e p o s i t i o n s o f S i 0 , break ing vacuum i n between t o t h o r o u g h l y c l e a n t $ e surface, and thereby remove any p a r t i c l e s .

A n o t h e r s i g n i f i c a n t p r o b l e m l i e s w i t h m a k i n g a h igh-Tc contact t o t h e u n d e r l y i n g c o i 1. The d i f f i c u l t y stems f r o m t h e p r e s e n c e o f r e s i d u e s a t t h e i n t e r f a c e . Our s o l u t i o n c o n s i s t s o f i o n beam e t c h i n g a c o n t a c t window i n t h e Si02.. S i n c e t h e A-15 superconductors are damage s e n s i t i v e , t h e Tc o f t h e ma te r ia l benea th t he con tac t window i s reduced by the i o n beam e tch ing . However, t h i s damage i s annealed o u t a t 800°C when t h e top. superconductor i s d e p o s i t e d . ( T h i s a p p r o a c h r o u t i n e l y y i e l d s c r i t i c a l c u r r e n t s o f t h e c o n t a c t i n e x c e s s o f 10 mA.) A l t e r n a t i v e s o l u t i o n s t o e t c h i n g t h i s c o n t a c t window, such as l i f t o f f , were also sought. The drawback i s t h a t t h e s e i n t r o d u c e p h o t o r e s i s t , and hence p a r t i c l e c o n t a m i n a t i o n , a t a c r i t i c a l p o i n t i n t h e p r o c e s s i n g which has a d e t r i m e n t a l e f f e c t on t h e i n t e g r i t y of t h e i n s u l a t o r .

One add i t i ona l p rob lem, more s p e c i f i c t o o u r c i r c u i t , a r i s e s i n b r e a k i n g t h e t o p s u p e r c o n d u c t - i n g film o v e r t h e S iOz s tep-edge. S ince the top film m u s t c o v e r t h e e n t i r e i n p u t c o i l ( i n o r d e r t o s e r v e a s an e f f e c t i v e g r o u n d p l a n e ) , a c lean, sharp step i s r e q u i r e d o v e r r e l a t i v e l y l a r g e d i s t a n c e s ( - 1 11011). P a r t i c l e c o n t a m i n a t i o n , a l o n g w i t h t h e f a c t t h a t t h e u n d e r l y i n g s u r f a c e i s n o t p l a n a r , makes t h i s p a r t i c u l a r l y d i f f i c u l t . One s o l u t i o n i s t o s a c r i f i c e

a smal l amount o f c o u p l i n g and c u t a narrow swath i n t h e Nb$n/Cu b i l a y e r a l o n g t h e s t e p ( s e e F i g . 2 ) . More r e c e n t l y , a b e t t e r s o l u t i o n has been developed i n wh ich the 1600 A o f S i 0 i s i o n beam e tched f rom t w o d i r e c t i o n s . The i& beam i s a l i g n e d t o e t c h p a r a l l e l w i t h t h e d i r e c t i o n o f t h e s t e p a n d 800 A o f Si02 i s removed. The sample i s t h e n r o t a t e d 180" and the rema in ing 800 A o f S i 0 2 i s removed. T h i s produces a c lean break i n t h e s u p e r c o n d u c t o r a l l a l o n g the s tep -edge . I ron i ca l l y , t he ang le evapora t i on o f t h e s u p e r c o n d u c t o r , a l o n g w i t h t h e s t e p edges un in ten - t i o n a l l y p r o d u c e d by t h e i n p u t c o i l , c r e a t e s d i f f i c u l - t i e s f o r t h e s e c t i o n o f t h e c o i 1 c o n t a i n e d i n t h e t o p l e v e l . To remain continuous, it must cross perpen- d i c u l a r t o t h e u n d e r l y i n g c o i l w i n d i n g s .

80 r

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R.0.18 a T= 4.2 K

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W b- a 8

I I I I I I I 1 I 0 100 2 0 0 300 4 0 0 500 600 700 000 900

I i o s CURRENT (FA)

Fig. 4 Cur ren t - vo l tage (I-V) c h a r a c t e r i s t i c f o r a c i r c u i t o p e r a t i n g a t 4.2 K. The two I-V curves show t h e maximum m o d u l a t i o n i n t h e c r i t i c a l c u r r e n t due t o a c u r r e n t a p p l i e d t h r o u g h t h e i n p u t c o i l . The i n s e t shows t h e vo l tage modu la t ion where the SQUID i s b i a s e d as shown.

SAMPLE G84-51 84 R = 0.180 T = 10.1 K

0 IO t 20 30 4 0 50 60 10 80 90 Ibias CURRENT ( p A 1

Fig. 5 C u r r e n t - v o l t a g e c h a r a c t e r i s t i c f o r same c i r c u i t o p e r a t i n g a t 10.1 K which i s n e a r t h e Tc of t h e SQUID. The I-V i s t o o n o i s e rounded t o observe a c r i t i c a l c u r r e n t a t t h i s temperature.

535

s u p p o r t t h i s p o s s i b i l i t y . F u r t h e r s t u d y will be r e q u i r e d t o e x p l a i n t h i s b e h a v i o r .

SUMMARY

We h a v e s u c c e s s f u l l y f a b r i c a t e d a h igh-Tc i n t e g r a t e d c i r c u i t c o n s i s t i n g o f a dc SQUID w i t h an i n p u t c o u p l i n g c o i l . The p r o c e s s d e a l s e f f e c t i v e l y w i th t he p rob lems i nhe ren t t o t he use o f h igh -Tc superconduc t ing ma te r ia l s and SNS Josephson micro- bridges. Problems s t i l l remain with' unexpectedly low c r i t i c a l c u r r e n t m o d u l a t i o n . However, by u t i l i z i n g a sma l le r coup l i ng ho le , t he c i r cu i t s ope ra te success - f u l l y o v e r a wide temperature range (so f a r , f r o m below 4.2 K t o n e a r l y 14 K). The measured value o f the mutual inductance i s c l o s e t o t h a t e x p e c t e d f r o m cons ide ra t i ons o f t he des igned geomet r i ca l i nduc t - ance. The des ign should be more g e n e r a l l y a p p l i c a b l e t o o t h e r t y p e s o f h i g h - T c i n t e g r a t e d c i r c u i t s .

ACKNOWLEDGEMENTS

We would l i k e t o t h a n k F. He l lman fo r sha r ing he r e x p e r t i s e with Nb2Sn f i lms, and A. de Lozanne f o r

IV. ELECTRICAL CHARACTERISTICS

C u r r e n t v o l t a g e c h a r a c t e r i s t i c s f o r a r e p r e - s e n t a t i v e dc SQUID w i t h a 5 0 - t u r n i n p u t c o i l o p e r a t i n g a t 4.2 and 10.1 K a r e shown i n F igs. 4 and 5, respec- t i v e l y . The SQUID has a r e s i s t a n c e o f 0.18 il c o r r e s p o n d i n g t o r e s i s t a n c e s o f - 0.36 i n each mic robr idge. The I R p r o d u c t f o r t h i s p a r t i c u l a r d e v i c e i s 20 pV. f h e i n s e t i n each o f t h e f i g u r e s shows the vo l tage modu la t i on o f t h e SQUID as a f u n c t i o n ' o f t h e c u r r e n t t h r o u g h t h e i n p u t c o i l . . C l e a r l y t h e c i r c u i t f u n c t i o n s o v e r a wide temperature range. The o p e r a t i o n i s n o t p e r f e c t , h o w e v e r . The ICR p r o d u c t i s low, r e s u l t i n g f r o m a lower Tc i n t h e m i c r o b r i d g e o f t h i s p a r t i c u l a r c i r c u i t . Such low Tc'S have been observed i n a number o f c i r c u i t s and are b e l i e v e d t o be due t o damage caused by t h e i o n beam e t c h i n g when t h e m i c r o b r i d g e s a r e n a r r o w e d t o submic ron w id ths . Fo r tuna te l y , t h i s p rob lem does n o t seem t o be i n h e r e n t i n o u r f a b r i c a t i o n p r o c e s s , as we h a v e o b t a i n e d h i g h e r T c c i r c u i t s w i t h I c R p roduc ts i n excess o f 0.5 mV a t 4.2 K.

The nu tua l induc tance, M y between the SQUID and i n p u t c o i l was determined by m e a s u r i n g t h e p e r i o d o f t h e SQUID vo l tage modu la t ion over a la rge range o f i n p u t c o i l c u r r e n t . F o r t h e SQUID shown,M = 0.20 nH and i s independent o f temperature as expected (see F ig. 4 and 5). The induc tance o f t he i npu t co i 1 was n o t measured. The geomet r i ca l i nduc tance o f t he SQUID due t o t h e l o o p ( d e t e r m i n e d , by t h e h o l e s i z e ) i s s i m p l y M/n,where n i s t h e number o f t u r n s i n t h e c o i l . The measured value f o r t h i s SQUID i s 4 pH. For comparison, the value o f t h e i n d u c t a n c e i n f e r r e d f rom the v I t a .e a t w h i c h t h e m o d u l a t i o n i s a t t e n u a t e d i s - 7 pH? 'Both of these inductance va lues are i n genera l agreement wi th that expected (7.8 pH) f o r t h e

5 Vm d iamete r coup1 i ng ho le t ha t was used i n t h i s c i r c u i t . I n c o n t r a s t , t h e t o t a l SQUID s e l f i n d u c t a n c e ( w h i c h i n c l u d e s t h e p a r a s i t i c i n d u c t a n c e due t o t h e m i c r o b i d g e s t r u c t u r e ) i s rmch larger, namely 39 pH. T h i s t o t a l SQUID s e l f i n d u c t a n c e i s i n f e r p e d f r o m t h depth o f t h e m o d u l a t i o n o f t h e c r i t i c a l c u r r e n t s , assuming equal c r i t i c a l c u r r e n t s i n t h e m i c r o - br idges.

%

As i n d i c a t e d by t h e above, a s i g n i f i c a n t p r o b l e m w i t h t h e c i r c u i t s i s t h e l o w d e p t h o f m o d u l a t i o n w h i c h i s w e l l be low that expected on t h e b a s i s o f t h e d e s i g n . I n o r d e r t o o b t a i n an observable depth o f modu la t ion i t has been necessary t o reduce the d iamete r o f t he coup l i ng ho le f rom the des igned va lue o f 60 um t o 5 urn. T h i s r e d u c e s t h e t o t a l s e l f i n d u c t a n c e o f t h e SQUID and hence t h e s e l f s h i e l d i n g . The reason f o r t h e p r o b l e m i s n o t understood but has beeq noted by o ther researchers us ing h igh -Tc ma te r ia l s .

A l t h o u g h p a r t o f t h i s d i f f i c u l t y 'may stem from unequal c r i t i c a l ' c u r r e n t s i n t h e i n d i v i d u a l m i c r o - b r i d g e s ( l e a d i n g t o an e r r o n e o u s l y h i g h i n f e r r e d v a l u e o f t h e t o t a l s e l f i n d u c t a n c e ) , t h i s c a n n o t be t h e pr imary problem. A c o m p l e t e d c i r c u i t with a 5 m coup l i ng ho le d iamete r was f i r s t measured and t h e n f u r t h e r p r o c e s s e d i n o r d e r t o d o u b l e t h e d i a m e t e r o f t h e c o u p l i n g h o l e ( w i t h o u t a l t e r i n g any o t h e r p a r t o f t h e c i r c u i t ) . When t h e c i r c u i t was remeasured the geomet r ica l induc tance was found t o s c a l e as pre- d i c t e d w h i l e t h e t o t a l i n d u c t a n c e was f o u n d t o i n c r e a s e by much more than expected.' The reason f o r t h i s remains unclear.

many he lp fu l d iscus2 ions . Th is work was suppor ted by t h e NSF-ECS.

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4. 5.

6.

7.

REFERENCES

A. de Lozanne, M.S. D i I o r i o , and M.R. Beasley, Appl. Phys. Let t . , 42, 541, (1983). M.S. D i I o r i o , A. deLozanne , and M.R. Beasley, IEEE Trans. Mag., MAG-19, 308, (1983). J.M. Jaycox and M;B. Ketchen, IEEE Trans. Mag.,

S e e g . 1 o f Kef. 3. J. Clarke, W.M. Goubau, and M.B. Ketchen, J. Low Temp. Phys.; 25, 99, (1976). C.D. Tesche and 4. Clarke, J. Low Temp. Phys. 29, 301 (1977). J.H. Claassen, Appl. Phys. Lett., 4 839, (1982). See a l s o R.F. Voss, R.B. LaibowTiz, and

A.N. Broers, Appl. Phys. Lett., 37, 656, (1980).

MAG-19,' 400, (1981).

Hence, a l t h o u g h t h e c i r c u i t f u n c t i o n s i n a l l regards, i t s performance has been degraded by t h e a p p a r e n t l y l a r g e t o t a l SQUID inductances. A l a r g e k i n e t i c i n d u c t a n c e s m i g h t a c c o u n t f o r such behav io r b u t a p r e l i m i n a r y e s t i m a t e o f i t s v a l u e does no t